+* create a btrfs qgroup for /var/lib/machines, and add all container
+ subvolumes we create to it.
+
+* nspawn: add --overlay= to support overlay file systems, similar to
+ --tmpfs= and --bind=.
+
+* When logging about multiple units (stopping BoundTo units, conflicts, etc.),
+ log both units as UNIT=, so that journalctl -u triggers on both.
+